Typically, after a well for the production of oil and/or gas has been drilled, casing will be lowered into and cemented in the well. During cementing, cement is forced down the bore of the casing, through an aperture in the guide shoe at the bottom of the casing, and up the annulus surrounding the casing and between the casing and the wellbore to the desired level. One or more valves, commonly termed float valves, are installed in the casing to prevent back flow of the cement into the casing from the annulus if pressure in the casing is reduced. Such a float valve may be in the form of a collar or an integral part of the guide shoe. The closed float valve or valves also seal the bottom of the casing and prevent fluids in the wellbore from filling it when the casing is lowered into the wellbore.
Some offshore applications and in particular, shallow water applications, have a requirement to maintain a full column of weighted fluid (typically drilling fluid or drilling mud), inside the casing string while running it from the rig floor to the sea-floor and into the borehole in riserless applications. Running the casing string full aids in getting the casing to the borehole in a controlled manner, helps to prevent kick and minimizes fluid contamination of wellbore fluids in the well. Kick is a condition where there is an influx of formation fluids into the wellbore. It occurs because the hydrostatic pressure exerted by the column of fluid contained within the wellbore and the drilling riser is not great enough to overcome the pressure exerted by the fluids in the formation drilled. Weighted fluids, such as drilling fluids, are heavier or denser than sea water and exert sufficient pressure to prevent kick. However, a common problem with offshore applications is that, during lowering of the casing to the borehole, the pressure differential between the drilling mud in the casing and the sea water surrounding the casing causes premature actuation of the float valve and allows sea water to displace the drilling mud. The sea water, being less dense than the drilling mud, exerts less of a hydrostatic pressure and thus, can allow kick to occur.
Past solutions to this problem have focused on increasing the activation pressure for the float valve; however, such techniques have proven to be problematic and impractical. Accordingly, it would be advantageous to provide a solution to this problem that did not involve increasing the activation pressure of the float valve.